The invention relates to a method and an apparatus for controlling the toner concentration of a dry developer, including a magnetic carrier and a non-magnetic toner, used in a dry type developing system.
In a dry type developing system for example, a magnetic brush developing system, the developer generally comprises a mixture of magnetic carrier and non-magnetic toner, which mixture is magnetically maintained for supply to an electrostatic latent image for converting it into a visual image. When the developer is supplied to the latent image, charged toner particles which adhere to carrier particles is attracted to the latent image by electrostatic interaction while the carrier remains attached to the developer retaining member under the influence of a magnetic force, so that after repeated developing process, only the toner component of the developer will be gradually reduced in quantity.
On the other hand, when converting an electrostatic latent image into a visual image by means of the magnetic brush developing system, it is known that the optimum toner concentration in the developer ranges from 3 to 5% by weight of the toner. Above 5%, the toner may be attrated to non-image areas, thus causing so-called back smearing in the visual image, while below 3%, the optical density of the visual image is reduced, resulting in a poor contrast. Therefore, it is necessary to maintain the toner concentration of the developer within a proper range in order to assure an optimum visual image.
In one technique of controlling the toner concentration, a change in the amount of reflected light from the developer as the proportion of the toner and carrier in the developer changes is utilized by irradiating the developer with light and determining the amount of reflected light therefrom to sense the toner concentration, thereby replenishing the toner so as to maintain a constant toner concentration. However, the described technique involves difficulties in that the light source and the light receiving element which are used for the purpose of photometric determination are liable to be soiled by the toner to result in varying photometric parameters and that a small difference in the reflectivity between the carrier and the toner results in an unsatisfactory sensing accuracy.
An alternative method is also known in which the toner is electrostatically attracted to the surface of an electrode plate to which a fixed voltage of the opposite polarity to that of the toner is applied, or to the surface of a dielectric member which is charged by an electric charge of the opposite polarity to that of the toner, followed by irradiation of the surface with light, and determining a change in the amount of light which has transmitted through or which is reflected from the surface to thereby detect the amount of toner attracted and hence the toner concentration in the developer. When this method is employed, the toner which is attracted to the surface must be wiped away each time the toner concentration is determined, resulting in the difficulty to detect the toner concentration on the continuous bases and in the susceptibility of the detection accuracy to the humidity.
A further alternative is to utilize a change in the apparent magnetic permeability of the developer as the proportion of the magnetic carrier and the non-magnetic toner in the developer varies. Thus, the developer is placed inside a coil to sense a change in the inductance of the coil, which is then converted into a voltage or the like, thereby determining the proportion of the carrier and toner in the developer. However, the device which is required to perform such a determination tends to be expensive. In addition, separate circulation path must be provided for the purpose of sampling independently from the inherent circulation path which is used for the purpose of developing, thus resulting in a complex overall system.